cucurbit powdery mildew

Symptoms

White powdery patches on upper leaf surface starting on older leaves, yellowing and shrivelling of affected leaves, premature senescence, sunscald on exposed fruit.

Plants affected

• melon
• begonia
• watermelon
• pumpkin (winter squash)
• cucumber
• butternut squash
• zucchini

Easily confused with

• cucurbit downy mildew

  How to tell them apart: Cucurbit powdery mildew (Podosphaera xanthii): white talcum-like patches mainly on the UPPER leaf surface that can be rubbed off, spreads in warm dry conditions. Cucurbit downy mildew (Pseudoperonospora cubensis): ANGULAR yellow patches on the upper surface bounded by the leaf veins, with a grey-purple fuzz on the underside, needs humidity/leaf wetness.

• sclerotinia stem rot

  How to tell them apart: Podosphaera xanthii (cucurbit powdery mildew) appears as dry, flour-like, white-to-grey powdery patches mainly on the upper and lower surfaces of leaves and on petioles; the coating is superficial, rubs off easily with a finger, and the leaf tissue beneath stays firm before later yellowing and drying. Sclerotinia sclerotiorum (white mould) instead forms dense, wet, cottony-fluffy white mycelium on stems, the crown, fruit and tissue near the soil line, accompanied by a soft, watery rot. The single most reliable sign is that Sclerotinia embeds hard, irregular black sclerotia (resembling mouse droppings) within or beneath its white cotton, which powdery mildew never produces. If you see dry dust on leaves it is powdery mildew; if you see slimy cotton on a collapsing stem with black lumps it is white mould.

• Cucumber mosaic virus (CMV)

  How to tell them apart: Cucumber mosaic virus (CMV) and Podosphaera xanthii (cucurbit powdery mildew) can both start as pale chlorotic patches on young squash leaves, but they diverge fast. CMV produces a mottled light-and-dark green mosaic spread through the leaf blade, often with puckering, downward cupping and shoestring narrowing of new growth, and the discolouration is IN the leaf tissue with nothing wiping off. Powdery mildew instead forms discrete round white-to-grey powdery colonies that sit ON TOP of the surface (usually upper leaves first) and smudge off when rubbed with a finger, the leaf yellowing only beneath the fungal mat. CMV symptoms are systemic and affect the whole plant including fruit deformation, while powdery mildew stays superficial and worsens in warm dry shaded conditions.

• silverleaf whitefly

  How to tell them apart: Bemisia tabaci (whitefly) shows tiny moth-like white adults that fly up in a cloud when the leaf is shaken, plus immobile pale-yellow scale-like nymphs stuck to the leaf underside; the white film wipes off and leaves sticky honeydew, and shaded lower leaves often blacken with sooty mould. Podosphaera xanthii (cucurbit powdery mildew) is instead a dry, talc-like white fungal growth that starts as round powdery spots on the UPPER leaf surface and stems, spreads into felty patches that do not fly away, and leaves no honeydew or insects. If nothing flies up and the coating is a dry mat of round spots, it is the mildew; if a white cloud rises and you find scale-like nymphs and sticky residue, it is the whitefly.

• grey mould (Botrytis)

  How to tell them apart: Both Podosphaera xanthii (begonia powdery mildew) and Botrytis cinerea (grey mould) can appear as a fungal 'fuzz' on begonia leaves and flowers, but they differ clearly on close inspection. Powdery mildew is a DRY, white-to-grey, powdery/flour-like film that sits on the surface, typically on the upper side of otherwise turgid green leaves, and wipes off cleanly with a finger leaving healthy tissue underneath; it thrives in mild, humid but dry-leaf conditions. Grey mould instead forms a GREY-to-tan, woolly, fuzzy growth on tissue that is already soft, water-soaked, brown and rotting (collapsed flowers, wounded petioles, dense wet foliage); it dislodges as a smoky cloud of spores when touched and is driven by free water and cool, very humid, poorly ventilated conditions. In short: dry white dust on living leaves = powdery mildew; grey fuzzy mould on wet, collapsing, rotten tissue = Botrytis.

Common questions

How do I recognise cucurbit powdery mildew on melon?

White powdery patches on upper leaf surface starting on older leaves, yellowing and shrivelling of affected leaves, premature senescence, sunscald on exposed fruit.

Do I have to report cucurbit powdery mildew?

No. cucurbit powdery mildew is not a notifiable organism, so there is nothing to report to an authority. Treat it as a normal garden problem.

How is cucurbit powdery mildew treated?

Mainly with biological and cultural measures — removing affected material, changing the conditions that favour it, and treating with agents approved in your country. The exact steps and their timing depend on the plant, the season and where you live: Dr. Phyto works them out in a free diagnosis.